1. Field of the Invention
This invention relates to solar selective coatings which can be painted onto suitable substrates which are formable after coating such as a bright aluminum substrate and to the formable, coated substrate. The coatings have sufficient toughness, flexibility and adhesion to enable the coated substrate to be post-formed into the absorber configuration desired for the solar heat collector.
2. Description of the Prior Art
It is known that the efficiency of flat-plate solar collectors can be increased if the solar absorptance of the surface of the collector is increased and the emittance of thermal radiation is decreased. Thus a flat-plate solar collector should have a "solar-selective" surface. This means that the absorptance .alpha. of the incident solar radiation with wavelengths ranging from about 0.3 .mu.m to about 2.5 .mu.m should be as high as possible and the emittance .epsilon. at longer infrared wavelengths of from about 2.5 .mu.m to about 30 .mu.m should be as low as possible.
Kokoropoulos, Salem and Daniels, Solar Energy 3 19 (1959) conclude:
"Many combinations of thin coatings of semi-conductors on highly reflecting metals can give surfaces which absorb more than 85 percent of the solar radiation at temperatures somewhat above 100.degree. C. and emit less than 20 percent of the infrared radiation emitted by an ideal blackbody radiator.
"For good selective radiation properties (i.e., high absorption in the solar region (0.3 to 2.5.mu.) and high transparency and low emissivity in infrared in the general region of 5 to 10.mu.) the coating must have a thickness (10.sup.-5 -10.sup.-4 cm) which is about equal to the wavelength of the radiation to be absorbed and about 1/10 or less of the infrared radiation which is emitted by the heated surface.
"The thin coating must be a black semiconductor. There is a close relation between the electrical conductance and the optical properties which in turn are determined by the atomic number and the unfilled electronic shells. The oxides of the transition elements are suitable for such selective radiation coatings. The greater the electrical conductance, the lower the emissivity. A thin film of cupric oxide has a lower emissivity than a thin film of cobalt oxide. The electrical properties of many of the semiconductors are well known, but the optical properties and the emissivity of thin films are less well known. The semiconductors, which increase in conductance as the temperature is raised, should give lower emissivities (in comparison with that of a perfect blackbody radiator) at higher temperatures."
A more recent report is "Optical Coatings for Flat Plate Solar Collectors", Honeywell, Inc., U.S. Dept. of Commerce Report PB-252 383--September 1975. This reports that coating thickness is most important factor in obtaining good selective properties and states that thicknesses less than 0.1 mil (2.5 .mu.m) are required. Meteor.RTM. 7890 (Harshaw) Cu-CrO.sub.x, is listed as one of the two best pigments of the 28 evaluated. About 30 binders were considered for known properties including fluoropolymers but none of the fluoropolymers were actually tested. Olefin binders were judged best. An example of a 0.05 mil (1.27 .mu.m) coating of Cu-CrO.sub.x /ethylene-propylene-diene with a pigment volume concentration of 30% on an aluminum substrate is reported to have an estimated .alpha..perspectiveto.0.92 and .epsilon..perspectiveto.0.30. All solar paint samples listed used ethylene-propylene-diene, polypropylene, silica, or polyethylene binders. The report indicates that fluorine-containing polymers are spectrally undesirable for this purpose because of the infrared absorption of the C-F bond over the 7-10 .mu.m range but no fluorine-containing polymers were tested. Despite this teaching away from the use of fluoropolymers, I have discovered how to make desirable post-formable solar selective coatings using certain fluorine containing polymers as binders, which binders are inherently adherent, weather-resistant and heat resistant. Thus it appears that the absorptions referred to in the art are inconsequential.